Current practices in the electric utility industry are increasingly seeing the necessity to install replacement transmission facilities, such as line insulators, while the line is energized. For example, a utility may have their personnel replace aged or contaminated porcelain or polymer insulators with new, replacement polymer insulators. The replacement of the existing insulator with the new polymer insulator, in some situations, may be done while the line remains energized (operating at its design voltage).
There have been incidents where a newly installed polymer insulator immediately failed when initially energized. For personnel safety purposes, utilities may require that the electrical integrity of each insulator be tested and verified before replacement under energized conditions. To accomplish this, the utility tests the insulators individually at a location remote from the installation site, such as at a high voltage test facility. The testing process is tedious, time consuming, and can only accommodate a limited number of insulators at a time.
Once the insulators are tested, they are tagged and sent to the worksite or stored for future use. Once the insulators leave the test facility, it is very possible for conditions to occur that may subsequently compromise the electrical integrity of the insulator. A safer and more appropriate test procedure would be to test the polymer insulators on-site immediately before installation.
FIG. 1 is a block diagram of an embodiment of a prior art test system. The prior art test device 102 is coupled to the polymer insulator 104 at the ground lead 106, via connection 108. The high voltage lead 110 is coupled to a high voltage source 112, typically at a value that corresponds to the intended operating voltage of the polymer insulator 104.
The polymer insulator 104 consists of a plurality of sheds 114 coupled together in a linear fashion as shown. The size of a shed 114 (diameter and thickness), and the number of sheds 114 used in any particular polymer insulator 104, may be designed such that the polymer insulator 104 may be used at a desirable operating voltage. Furthermore, great design flexibility is available such that the polymer insulator 104 may be connected to a power line structure (e.g.: pole, tower or the like) and the energized conductor in a variety of manners. It is appreciated that the simplified illustrative diagram of the polymer insulator 104 having five sheds 114 is indented for illustration purposes to demonstrate testing techniques and issues.
Prior art test devices 102 are not suitable for testing modern, high resistance, polymer insulators for “good-as-new” condition. Typically, prior art test devices 102 are used in a laboratory or test facility environment to test insulators with much larger leakage component currents.
During testing, a voltage is applied to the polymer insulator 104 at high voltage lead 110 such that an electric field (not illustrated) is established around the polymer insulator. The prior art test device detects leakage currents (Iinsulation, Istray and Isurface). If the polymer insulator 104 is properly functioning, the total detected leakage current will be less than a specified threshold. Here, currents Iinsulation is a leakage current component traveling through the internal portions of the polymer insulator 104. Istray are capacitive type leakage currents established through the surrounding air. Isurface is a leakage current flowing over the surface of the sheds 114.
One difficulty in the illustrated test system of FIG. 1 is that the polymer insulator 104, when tested, may be in close proximity to an object 116. Istray components are established between the proximity object 116 and the polymer insulator 104 during testing. Furthermore, various Istray components from the high voltage lead 110, the sheds 114, the prior art test device 102 and ground are established. There is no practical way to predict the magnitude or effect of these Istray components on the test result. Electrically, these random stray currents cause a phase shift in the total measured leakage current. These unpredictable Istray components, which cannot be easily measured or detected, may cause significant test result errors when testing modern polymer insulators that have an electrical impedance dominated by high resistance.
Furthermore, applying a high voltage to the high voltage test lead 110 may be dangerous to the testing personnel in the event of an accident. Typically, an accident during high voltage testing may significantly injure or may be fatal to the victim of the accident. Accordingly, testing at a test laboratory or test facility is difficult because of safety issues and the associated testing procedures that must be followed during the test.
It may also be desirable to test other electric system devices before installation. For example, protective devices such as metal oxide varistor (MOV) arresters are used to protect transmission facilities from sudden voltage surges, such as might be caused by a lightning strike or system disturbance. During transportation from the test site to the field location, the MOV arrester may become damaged if jarred or vibrated, as may occur when transported to a remote site over rough unimproved roads or trails. However, such damage may not be visibly apparent to the personnel installing the MOV arrester. Or, over time, the insulating characteristics of the MOV arrester may degrade before time to install in an energized line. In addition, MOV arresters degrade in service over time as a result of protecting transmission facilities from sudden voltage surges. If such defective or degraded MOV arresters remain in service, they may fail catastrophically or not be able to provide the intended protection to the electric transmission system. Upon failure during a voltage surge event, other system facilities may be damaged because the MOV arrester did not properly function.
MOV arresters may be tested at the remote facilities, much like the above-described polymer insulators 104. However, the testing process is tedious, time consuming, and can only accommodate a limited number of MOV arresters at a time.